What is the smoking gun signature of string theory? Suppose we have a complete and consistent model of quantum gravity with a zero or negative cosmological constant, but all we are given is its complete S-matrix. Using only this information, how do we figure out whether or not this model corresponds to a string theory compactification?

If the cosmological constant is positive, what can we use in place of the S-matrix?

@QMechanic - The question you link to is itself closed on account of being a duplicate of this question. But this question seems to be asking for something more narrowly defined (and therefore more reasonable) than the previous two.
–
user346May 1 '11 at 9:17

1

This is a meaningless question. One can't objectively say in advance what is "the smoking gun signature" of a theory - any theory. The term "smoking gun signature" is a term in the history of science, not science itself, which is associated with a particular piece of evidence that historically convinced most of the people that a new theory was established. However, in each case, "the" smoking gun signature could have been something entirely different, too. Each theory has many manifestations - and especially string theory has many more consequences than other theories.
–
Luboš MotlMay 1 '11 at 11:04

5

One can say dozens of very general things that could be observable at the Planck scale according to string theory. However, none of them has been observed yet for the simple reason that we can't observe Planckian phenomena directly in practice. It's much more likely that a low-energy fingerprint will be observed first, which is inevitably related highly indirectly to the fundamental theory, but we don't know which key aspect will be observed first and whether people will ever be able to observe one at all.
–
Luboš MotlMay 1 '11 at 11:06

Various alternatives to the Standard Model invoke new symmetries,
new forces or constituents. Furthermore, there are high hopes for discoveries that could pave the way toward a unified theory. These discoveries could take the form of supersymmetry or extra dimensions, the latter often requiring modification of gravity at the TeV scale.

So experimentalist have been lured to look for extra dimensions, one of the possible marks of string theory, as "predicted" by various phenomenological models.

If nothing new/unexpected like this is found, it will not preclude that string theories are waiting for us as the theory of everything in the future.

If a signal of extra dimensions is found in the data, then string theorists will be singing a different, not so cautious song.

Large extra dimensions aren't related to string theory. String theory's extra dimensions would be at the Planck scale, not the TeV scale.
–
Ben CrowellMay 12 '13 at 19:20

Now that I think about it, I think my comment above was an oversimplification. In theories with large extra dimensions, the Planck length as inferred from macroscopic experiments isn't the true Planck length; the motivation is that the true Planck length is the same as the electroweak scale, but we see the Planck length in a distorted way because we don't see the extra dimensions. But I think it's still not true that the discovery of large extra dimensions would be evidence in favor of string theory.
–
Ben CrowellMay 14 '13 at 4:21

@BenCrowell At the time the LHC experiments were planned the string phenomenologists did propose it as a smoking gun of string theory, and as such it was adopted by the planners of the experiments back in 2000 or so. It was a series of papers by Dimopoulos and Arkani Hammed et al .Later a different proposal appeared but at the moment I cannot remember the authors. After all it is string theories that introduced extra dimensions so if they are found as accessible, the first thought would be "phenomenologists were correct". Maybe later alternatives would appear.
–
anna vMay 14 '13 at 4:50